An aluminum alloy is lightweight and has high heat conductivity, and thus is used in a heat exchanger for an automobile, for example, a radiator, a condenser, an evaporator, a heater, or an intercooler. The heat exchanger for an automobile is mainly produced via a brazing method. In general, brazing is conducted at a high temperature of about 600° C., by using an Al—Si-based filler alloy. Thus, an aluminum alloy brazing sheet, having excellent brazing properties and having high strength after brazing and corrosion resistance, is required.
An aluminum alloy heat exchanger to be produced through brazing is formed of a corrugated fin for mainly conducting heat radiation and a tube for circulating cooling water or a cooling medium. In the case where the tube is penetrated through corrosion or fracture, the cooling water or cooling medium circulating inside the tube leaks. Thus, an aluminum alloy brazing sheet having excellent strength after brazing and corrosion resistance, is essential for extending a life of a product using the same.
In recent years, a demand for reduction in weight of an automobile has been increasing, and corresponding reduction in weight of an automobile heat exchanger has been required. Thus, reduction in thickness of each member forming the heat exchanger has been studied, since there is a need of an aluminum alloy brazing sheet having further improved strength after brazing and corrosion resistance.
Hitherto, a tube material of a heat exchanger in which cooling water circulates inside the tube, such as a radiator or a heater for an automobile, generally employs a three-layer tube material obtained by: cladding a sacrificial anode material, such as an Al—Zn-based alloy, on an inner surface of a core alloy, such as an Al—Mn-based alloy typified by a JIS 3003 alloy; and cladding a filler alloy, such as an Al—Si-based alloy, on an atmospheric side of the core alloy.
However, the mechanical strength after brazing of the clad material employing the JIS 3003 core alloy is about 110 MPa, which is insufficient.
For improving the strength after brazing, a three-layer clad tube material having Mg added to a core alloy is proposed (see JP-A-8-246117 (“JP-A” means unexamined published Japanese patent application), and JP-A-2003-55727, for example). However, the addition of Mg to the core alloy causes a reaction of a fluoride-based flux to be used in a Nocolok brazing method and Mg, leading to formation of a compound such as MgF, and thereby to significantly degrade brazing properties.
Further, there is proposed a tube material capable of preventing degradation of the brazing properties and having improved strength, which is obtained by adding Mg to a sacrificial anode material cladded to an inner surface of the tube, rather than to the core alloy (see JP-A-6-212332 and JP-8-283891, for example). Since the tube material contains Mg added to the sacrificial anode material, the tube material can be used as a material for a seam welded flat tube in which the sacrificial anode material is not in contact with a bonding surface, but the tube material cannot be used as a material for a flat tube in which a surface of the sacrificial anode material must be bonded through brazing.
Further, there is proposed a four-layer clad material in which an intermediate material is formed between a core alloy and a filler alloy of a three-layer clad material formed of a filler alloy, a core alloy, and a sacrificial anode material (see JP-A-6-73480, JP-T-2005-505421 (“JP-T” means published searched patent publication), and JP-A-2005-161383, for example). The four-layer clad material is improved in strength after brazing by using a material containing a large Mg content in the core alloy. Further, diffusion of Mg, which is added to the core alloy, into the filler alloy is suppressed by forming the intermediate material having a low Mg content between the filler alloy and the core alloy. Thus, degradation in brazing properties is prevented in the case where a Nocolok brazing method is used for the four-layer clad material.